1. Field of the Invention
The present invention relates generally to a method and apparatus for controlling a refrigeration system and, more particularly, to a method and apparatus for economizing the operation of an absorption, centrifugal or other type refrigeration system.
2. Description of the Prior Art
In most large air conditioned buldings, the energy source for the air conditioning system is the largest single consumer of energy. Thus, projected utility costs play a large role in determining the specifications of a new building's heating and cooling plant. Most large commercial structures employ absorption or centrifugal type refrigeration systems. In the case of absorption-type refrigeration systems, fuel to generate steam heat is the primary energy consumer. In the case of centrifugal-type refrigeration systems, the electrically-driven compressor motor is the primary energy consumer. At present, most chillers are programmed to operate and control an air-conditioning system by chilling water to a constant predetermined temperature to meet the air conditioning needs on the hottest day of the year. This arrangement results in substantial system over capacity much of the time with attendant inefficiencies and increased utility or energy costs. Chiller controls that automatically adjust the chilled water temperature to meet the requirements of a cooling loop on a moment-to-moment basis have been proposed and can substantially reduce total system power consumption.
Centrifugal cold-type refigeration systems for chilling a fluid medium which may then be used to provide refrigeration to any desired location are well known. In a typical centrifugal refrigeration system, a high-pressure refrigerant vapor is discharged from a compressor in a super-heated state and enters a condenser. In the condenser, the refrigerant vapor is condensed, essentially at constant pressure, by giving up its latent heat to the cooling water flowing through a heat exchanger or to the atmosphere. The saturated liquid refrigerant is collected in a receiver tank. Thereafter, the saturated liquid refrigerant enters an expansion valve or throttling valve at high pressure. The expansion valve effects a substantial pressure reduction in the refrigerant. Simultaneously, with the reduction in pressure, an associated reduction in temperature takes place, the new temperature corresponding to the boiling temperature at this lower pressure. The actual mechanism accounting for this lowering of the liquid refrigerant temperature consists of the flashing into vapor of a portion of the liquid and, as there exists no external source of heat, the energy for this evaporation is supplied by the liquid refrigerant itself, thus causing its temperature to drop. The low-temperature liquid refrigerant, with a small fraction of its vapor, is then admitted to a heat exchanger called an evaporator. Within the evaporator, the liquid refrigerant is evaporated by heat transferred to it from the comparatively warmer space to be refrigerated. The flow rate of the refrigerant is so adjusted by the thermostatic expansion valve that at the exit of the evaporator all the liquid refrigerant is in the saturated vapor state. The vapor leaving the evaporator enters the suction side of the compressor and is compressed to a higher pressure. The work of compression raises both the pressure and temperature of the refrigerant vapor so that it is discharged in the superheated vapor state and ready to repeat the entire cycle over again.
Efficient operation of the heat exchanger is extremely important to the economic cycling of the refrigerant within the centrifugal refrigeration system. Typically, centrifugal refrigeration systems are controlled by providing a temperature sensor which senses the temperature of the fluid medium generally as it leaves the evaporator. The signal from the sensor is then used to control the capacity of the refrigeration system according to its predetermined constant fluid medium temperature output. In a centrifugal-type refrigeration system, this is accomplished by controlling the compressor's inlet vane geometry by techniques well known in the art.
Absorption cold-type refrigeration systems for chilling a fluid medium which may then be used to provide refrigeration to any desired location are also well known. In a typical absorption refrigeration system, a liquid chilled in an evaporator is circulated to a refrigeration load, such as one or more remotely located air conditioning units or cooling system for an industrial process. Typically, the system includes a concentrator section, an evaporator section, a condenser section, an absorber section and a heat exchanger. In the concentrator section, heat energy from steam or hot water is used to boil a dilute solution of lithium bromide and water. This boiling results in release of water vapor, and in concentration of the remaining lithium bromide solution. The water, or refrigerant vapor released in the concentrator, is drawn into the condenser section. Cooling tower water flowing through the condenser tubes cools and condenses the refrigerant. The refrigerant flows through an orifice into the evaporator section. The pressure in the evaporator section is low, corresponding to a refrigerant saturation temperature of approximately 40.degree. F. The refrigerant is sprayed over a tube bundle containing building system water. The system water gives up heat to the refrigerant, causing it to vaporize. Water is frequently used as the refrigerant, the chilled fluid, and the cooling media, and the source of heat in a generator may be steam or hot water circulated in a heating coil. Typically, when the refrigerant is water, the absorbent solution is a hygroscopic brine, such as an aqueous solution of lithium bromide or lithium chloride. As is well known, however, a large number of fluids with widely varying characteristics may be used in absorption refrigeration systems.
The refrigerant vapor is drawn to the absorber section by the low pressure resulting from absorption of the refrigerant into the absorbent. In order to expose a large amount of lithium bromide solution surface to the water vapor, the solution is sprayed over the absorber tube bundle. The absorber cooling coil is connected in a cooling circuit which usually also includes the condenser coil conveniently situated in series with the cooling coil. The cooling water is conducted from the condenser coil to a remotely located cooling tower where the heat abstracted in the absorber and in the condenser is rejected to ambient air; that is, the air out-of-doors. Cooling tower water is used in the tube bundle within the absorber section to remove the heat of absorption that is released when the refrigerant vapor returns to the liquid state. The degree of affinity of the absorbent for refrigerant vapor is a function of the absorbent solution concentration and of temperature. The more concentrated the solution and the cooler the solution, the greater the affinity for refrigerant vapor. Consequentially, the pressure, and thereby the saturation temperature, in the evaporator is controlled by the concentration of lithium bromide solution in the absorber. The concentration of this solution is determined by the amount of heat applied in the concentrator section of the machine. As the absorbent absorbs refrigerant vapor, the solution becomes increasingly dilute. It is necessary to continuously circulate this dilute solution back to the concentrator in order to keep the cycle continuous.
A heat exchanger exchanges heat between the relatively cool, dilute solution being transferred from the absorber to the concentrator section with a hot, concentrated solution being returned from the concentrator to the absorber. Transferring heat from the concentrated solution to the dilute solution reduces the amount of heat that must be added to bring the dilute solution to a boil. Simultaneously, reducing the temperature of the concentrated solution reduces the amount of heat that must be removed from the absorber section in order to obtain the desired absorber efficiency. Efficient operation of the heat exchanger is extremely important to the economic operation of the lithium bromide water cycle. Typically, absorption refrigeration systems are controlled by providing a temperature sensor which senses the temperature of the fluid medium as it leaves the evaporator, as in the case of the centrifugal refrigeration system. The signal from the sensor is then used to control the capacity of the refrigeration system according to its predetermined constant fluid medium temperature output. In an absorption-type refrigeration system, this is accomplished by controlling the concentration of the refrigerant-absorbent solution supplied to the absorber.
Other control systems, such as those disclosed in U.S. Pat. Nos. 3,099,139 and 3,250,084 employ two separate temperature sensors, one disposed to sense the temperature of the fluid medium entering the refrigeration system, and another to sense the temperature of the fluid medium after it has been cooled by the refrigeration system. However, these control systems, as well as the other prior art control systems known to the Applicant, are only adapted to vary the refrigeration capacity of the system in response to varying load conditions and not in response to changes in the internal conditions in the refrigeration system itself. The control system described in U.S. Pat. No. 4,090,372 attempts to solve this problem by providing means disposed for sensing variations in the temperature of the fluid medium both as it enters and leaves the evaporator and for sensing variations in the temperature in the absorber. This sensing means provides first, second and third tracking signal outputs indicative of the sensed temperature variations, the first tracking signal output being indicative of the variations in temperature of the fluid medium at the evaporator inlet, the second tracking signal output being indicative of the variations in temperature of the fluid medium at the evaporator outlet; and the third tracking signal output signal being indicative of the variations in temperature within the absorber. Comparator means are utilized for comparing the signals and providing a first control signal output indicative of the selected tracking signal. A second control signal output is generated dependent on the relative values of the second tracking signal and the first control signal. The first and second tracking signal outputs are also operatively connected to a difference means which provides a third control signal output indicative of the difference between the first and second tracking signals. The control system also includes a second comparator means operatively connected to the second and third control signal outputs for comparing the second and third control signals and selecting one of them based upon its relative value as compared with the other and for providing a fourth signal output indicative of the selected control value. This fourth signal control is then used to control the components of the refrigeration system which controls the concentration of the refrigerant absorbent solution supplied to the absorber. This complicated control system operates to reduce the fuel input to the refrigeration system whenever the fuel is being inefficiently used due to changes in internal operating temperatures or other criteria of the refrigeration system. Further, this complicated control system is designed to vary the output chilled water temperature in response to changes in load, thereby further reducing fuel consumption of the refrigeration system.
All of the above-discussed prior art control systems direct themselves to developing control devices of varying degrees of sophistication for regulating the operation of the absorption refrigeration system controlling parameters within the system itself. Even with these sophisticated internal controls, the system will often times generate excess chilled water for circulation throughout the building which has been lowered to a temperature below the comfort point thereby requiring auxiliary heating systems to automatically provide heat in order to compensate for the excessive cooling. In effect, the absorption type refrigeration system of the type used in large building complexes relies upon an auxiliary heating system in order to provide comfortable conditioned air for an installation such as a large hospital.
U.S. Pat. No. 4,270,361 to LaBarge discloses a controller for a centrifugal type water chiller which overcomes some of the shortcomings of the prior art in the area of optimized energy management. In LaBarge, an automatic chiller control provides a temperature sensitive load limit compressor control on a cooling water system, such as is used in buildings for air conditioning. The controller operates across at least one motor driven compressor and operates to adjust the vane inlet of a typical centrifugal compressor. Control is provided in discrete steps responsive to changes in cooling water temperature as the water typically returns from a building. The control, when operated in the automatic position, starts the chiller at minimum possible power consumption. Increasing load or unload of the chiller is provided in incremental steps (for example, 5% of full load) at preselected time intervals (for example, ten minutes) which are typically adjustable to any other desired time span. The control seeks the required motor power output to improve operating efficiency. Provision is made where the controller is operating the chiller at less than an efficient level (for example, with vanes closed) to periodically shut the chiller down for an adjustable time as required. Options of overriding manual adjustment as well as manual adjustment only are also disclosed.
Although devices, such as that disclosed in LaBarge, represent improvements over the theretofore prior art devices to date, no prior art devices have attempted to continuously control the refrigeration system on a real time basis as a function of the temperature of chilled water returning from the load or building. Instead, LaBarge only controls operation of the water chiller during select portions of the duty cycle, such as start-up, and only operates intermittently, after pauses of predetermined duration.
Additionally, none of the prior art devices to date have attempted to introduce a control signal directly to the refrigeration system which controls the amount of refrigerant or chilled water produced as a function of outdoor air temperature and outdoor humidity. In other words, the prior art devices have failed to recognize that the refrigeration system chilled water output is a function not only of the demands of the system's internal characteristics for generating a set quantity of chilled water but, also a function of the outdoor humidity and outdoor temperature.
Although the present invention is described as embodied in an absorption-type refrigeration system, it is to be understood that it can be applied with equal success in centrifugal or other types of refrigeration as will be apparent to those of ordinary skill in the art in view of the present specification. Additionally, although described as being applied to a single load, it is contemplated that it can be employed with multiple or ganged loads with only minor modification. Finally, although described and primarily intended for large institutional-type structures, such as hospitals and office complexes where precise, high performance, versatile and cost-efficient heating/cooling plants are imperative, in its broadest sense, the device described herein can also be applied to many other types of loads.